A breeding program designed both to produce a definitive version of the D2.Fv-2r congenic strain and to confirm our earlier finding of lethality of the Fv-2r gene on the DBA/2 genetic background will be pursued intensively. This work should progress more rapidly than it has in the past, since we have confirmed that the Bgs locus, closely linked to Fv-2, can be used in these crosses as a marker. We will also pursue the study of an antiserum recently obtained which appears to identify an antigen(s) specific for the SFFV component of FV. This serum was prepared by immunizing Fisher rats with a line of Fisher rat embryo cells infected with SFFV but not MuLV.